1. Field of the Invention
The invention in the field of thermal-storage devices that are suitable for use in air-conditioning systems.
2. Description of Prior Art
Thermal storage for space cooling and heating has a long history. Passive systems for buildings using massive masonry walls have been used for millennia in dry climates to provide a more-even interior temperature despite large swings in ambient temperatures.
Active thermal storage has been used for solar heating systems. Two types of thermal storage are common in these systems. The first type is storage use using insulated tanks of water. Water has the advantage of having high specific heat and the ability to store a large amount of heat in small volume, which reduces storage size. Unfortunately, while water itself is inexpensive, the insulated storage tanks necessary for holding the water and maintaining its temperature are quite expensive, typically a $1 per gallon or more. A secondary problem is water mixes very easily in a storage tank, which makes it difficult to achieve thermal stratification. Stratification is desirable for maximizing thermal storage capacity.
A second type of storage circulates air through a pebble bed. This type of storage takes more space than water storage, but it does not require a storage tank. The stone necessary for a pebble bed has a significant cost and the space requirements mean that this type of storage can take a large portion of a basement or other part of building. Because of these limitations, this type of storage is normally used for systems where air is the fluid circulating through the solar collectors since it eliminates the need for water-to-air heat exchangers.
More recently much work has gone into thermal storage for cooling systems. Thermal storage is used to allow the use of low-cost, off-peak electricity to run cooling systems at night to provide cooling necessary during the day.
Two types of storage are common for this application. First is cold-water storage, which simply stores chilled water in an insulated storage tank. In addition to the problems discussed above, the size requirements of the tank can be quite large because of the limited temperature differential available between the freezing point of water and the temperature required for useful cooling and dehumidification of the space (typically ˜20° F.).
A second option for thermal storage for cooling is ice. Ice storage has the advantage of using the heat of fusion for water, which allows for a high storage density. Unfortunately making ice requires low evaporating temperatures from the refrigeration systems to reach the freezing point of water and to overcome the thermal resistance of ice that forms on heat exchanger surfaces. These problems reduce the efficiency and increase the cost for ice storage.
Phase-change systems have also been developed using various kinds of salts or other materials that freeze at appropriate temperatures for heating or cooling purposes. Unfortunately, these materials are usually quite expensive and frequently have problems very slow freezing rates, which create additional problems beyond those found with ice storage. Toxicity of materials can also be a concern in some cases.
Gravel-liquid mixtures have seen same use in thermal applications in the prior art. Harrison (U.S. Pat. No. 4,010,731) describes a system for storing high-temperature thermal energy from a solar collector using a buried, insulated tank containing a mixture of stones and water. The system does not contain any means for storing cooling capacity and teaches that a much different design that uses ice is required for storing cooling capacity.
Hanchar (U.S. Pat. No. 5,477,703) is an example of a system that uses gravel and water in an underground enclosure to transfer thermal energy between a ground-source heat pump and the ground. This arrangement does not allow direct use of water from the enclosure for directly cooling a building, and instead proposes rejecting heat from the heat pump to the enclosure during the cooling season.
Ippolito (U.S. Pat. No. 4,392,531) describes a similar arrangement that uses wells that contain a mixture of water and gravel for providing thermal input to a heat pump. Ippolito teaches the use of a separate, conventional tank for transient storage of thermal energy that is then used for space heating and cooling.